Transcript 1. TCP over CNN

TCP over Constrained-Node Networks
draft-gomez-lwig-tcp-constrainednode-networks-01
Carles Gomez
Universitat Politècnica de Catalunya (UPC)/Fundació i2cat
[email protected]
Jon Crowcroft
University of Cambridge
[email protected]
IETF 97 – Seoul, November 2016
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Status
• draft-gomez-core-tcp-constrainednode-networks-00
– Presented in Berlin (LWIG and TCPM WGs)
• draft-gomez-lwig-tcp-constrainednode-networks-00
– Not modified
• draft-gomez-lwig-tcp-constrainednode-networks-01
– Several updates
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Motivation
• Several application layer protocols being used for
the Internet of Things (IoT)
– Constrained Application Protocol (CoAP)
• Originally over UDP
• CoAP over TCP in progress
– To overcome middlebox problems
– HTTP/2 and HTTP/1.1
– XMPP
– MQTT
TCP
• TCP is being / will be used in many IoT scenarios
– Offer simple measures for suitable TCP
implementation/operation over CNNs
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Updates in -01 (I/X)
• RFC 2119 language removed
– Purpose is to document how TCP can be used in
CNNs (i.e. not proposing a new TCP variant)
• 1. Introduction
– XMPP added to the list of protocols used in CNNs
that use TCP
• 2. Characteristics of CNNs relevant for TCP
– Often, low transmission rates (typically < 1 Mbit/s)
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Updates in -01 (II/X)
• 3. Scenario
– Asymmetric
• Resource availability
• Constrained devices: data sent > data received
– Majority of constrained devices will be sensors
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Updates in -01 (III/X)
• 4.1. TCP connection initiation
– Typically initiated by the constrained device
• Better support sleep periods in some RDC techniques
• 4.2. Maximum Segment Size
– If a link layer offers MTU > 1280 bytes, still set
MSS so that IPv6 datagram size ≤ 1280 bytes
• Avoid issues with Internet links with not as high MTU
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Updates in -01 (IV/X)
• 4.3. Window Size
– Recommends single-MSS window
• Receive and transmit
– If device can afford it, 5-MSS window may be useful
• Allow Fast Retransmit and Fast Recovery
• 4.4. RTO
– draft-ietf-core-cocoa outperforms RFC 6298 RTObased improvements (Peak Hopper, Linux RTO)
– TBD: how to proceed?
• Using CoCoA for TCP conflicts with RFC 6298 and Karn
algorithm...
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Updates in -01 (V/X)
• 4.5. TCP connection lifetime
– Long TCP connection lifetime
• Minimize overhead
• However, firewalls may delete filter state records early
– TCP keep-alives not useful to solve the issue
– Short TCP connection lifetime
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•
•
TCP Fast Open (TFO) - RFC 7413
Data carried in SYN or SYN-ACK segments
Cookie must be obtained, included in SYNs and refreshed
More efficient than frequently opening new TCP
connections (with traditional 3WHS)
– As long as cookie refresh rate well below new connection rate
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Updates in -01 (VI/X)
• 4.7. TCP options
– For single-MSS receive/transmit window
• Not supporting, and ignoring if received
– Window Scale
– TCP timestamps
– SACK, SACK-Permitted
– For less constrained devices
• SACK avoids unnecessary retries, and reduces latency,
bandwidth and energy consumption
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Updates in -01 (VII/X)
• 4.8. Delayed Acknowledgments
– Device advertising single-MSS receive window needs to
avoid supporting delayed ACKs
• Avoid contributing up to 500 ms extra delay to the RTT
– Not recommended in CNNs
• Since traffic mostly of transactional type
• Transaction size often < MSS
– Could be useful to reduce # of ACKs in bulk transfers
• Infrequent, compared to rest of transactions
• 5. Security considerations
– If TFO used, security considerations of RFC 7413 apply
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Updates in -01 (VIII/X)
• Annex: TCP implementations for constrained
devices
– uIP
•
•
•
•
•
•
TCP/IP stack for 8- and 16-bit CPUs
Code size ~5 kB (checksumming, IP, ICMP and TCP)
Global, single-packet sized buffer for incoming packets
No buffer for outgoing data
MSS not modified during a connection
Stop-and-wait
– Avoids sliding window operations (32-bit arithmetic, expensive on
8-bit CPUs)
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Updates in -01 (IX/X)
• Annex: TCP implementations for constrained
devices
– lwIP
• TCP/IP stack for 8- and 16-bit CPUs
• Code size: ~14 kB to ~22 kB
– Memory management, checksumming, network interfaces, IP,
ICMP and TCP
– TCP code size: ~9 kB to ~14 kB
• Buffering of incoming and outgoing data
– Applications decoupled from the network stack
• Transmission window greater than a single segment
• Slow start, congestion avoidance, fast retransmit, fast
recovery
• SACK and Window Scale not implemented
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Updates in -01 (X/X)
• Annex: TCP implementations for constrained
devices
– RIOT
• TBD
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WG adoption ?
IETF 97 – Seoul, November 2016
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